Power line signaling



9 Mach 19, 1929.

H. B. ARNOLD .POWER LINE SIGNALING Filed Feb. 28, 1927 8 Sheets-Sheet 1 Marchv 19, 1929.

Filed Feb. 28, 1927 H. B. ARNOLD POWER LINE SIGNALING 8 Sheets-Sheet 2 `March 19, 1929. H. B'. ARNoLD 1,705,891

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Mms/NAL March 19, 1929. HI B, ARNOLD 11,705,891

POWER LINE SIGNALING I Filed Feb. 28, 1927 8 Sheets-Sheet 4 March 19, 1929. H. B. ARNOLDv 1,705,891 f POWER LINE .S IGNALING Filed Feb. 28, 1927 8 Sheets-Sheet 5 MAW/*Izzy 1 Mahl), 1929. H. B. ARNoLD Y POWER LINE S IGNALING Filed Feb. 2'8, 1927 e sheets-sheet e aan:

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POWER LINE! SIGNALING Filed Feb. 28, 1927 Y a sheets-shed v .SMT/olv C UNITED STATES HENRY B. ARNOLD, OF NEW YORK, N. Y.,

ASSIGNORTO BELL TELEPHONE LABORA- TORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF ANEW YORK.

.POWER LINE SIGNALING.

Application filed February 28, 1927. Serial No. 171,402.

I This invention relates to communication by means of carrier frequency waves, and particularly to communication over composite power current and carrier wave transmission systems employing repeaters. Y y

An object of the invention is toincrease the utility and eiliciency of power line signaling systems employing repeaters.

A related object of the invention is to establish independent signaling circuits on opposite sides of a` repeater in such-a system. j

Another objectof the invention is to apprise stations connectedto a signaling circuit on one side of a repeater when a signaling circuit on the opposite side of the repeater is in y service.

The invention provides va selectively operated power line signaling system employing :i plurality of stat-ions and a repeater, in which any station on the line may freely call and communicate with any other station or the repeater. A feature ofthe invention is an arrangement for establishing communication between stations located on one-sider of thc repeater without interrupting communication between stations located. on the opposite side of the repeater. Another feature of the invention is an arrangement whereby a hu sy signal is transmittedto'warn stations on one side of the repeater when stations on the ether side of the repeater are in service.k

il he various features of the invention will f he dcscrihedin detail in connection with the accompanying drawings, in which:

Figs. l to 8, inclusive,`illustrate diagram-v matically a power line signaling system comprising a plurality of stations and a repeater embodying the invention; and

Fig. 9` is a diagram showing the manner in which the severa-l sheets of drawings should he organized.

in. the system shown in thedrawings7 a plurality of stations and a. repeater are arranged for communication over a high voltage transmission line 10. By high voltage transmission hue is to he understood any system or r work of conductors for the electrical transmission ofpower over which it is possible to transmit high frequency waves.

The repeater, shown in Figs. 3 to 6, inclusive. may he en'iployed to repeat calling signals and telephone conversations Vbetween the stations either when the lpowerline l0 is a continuous line, or when a discontinuity, such as a voltage transformer or an open 'ment andfunction to that shown at station C in Figs. 7 and 8.

The repeater station'shown in Figs. 3 to 6, inclusive, includes a repeater R-W which receives waves originating at stations A and B on the west, or left han-d side of the repeater station, and a repeater R-E which receives waves originating at stations C and D on the east or rightliand side of the repeater station. The high frequency wave transmitting and receiving circuits employed atthe several stations are, in general, similar, and hence such apparatus as is common to, all of the stations will he described in detail only in connection with the description of station A.

quency, and stations D and E at the right of.

the repeater may be arranged to transmit or receive carrier waves of and 110 kilocyeles frequency. On outgoing calls stations A and B may he arranged to transmit at kilocycles and receive at 100 lilocycles, iny

which case on incoming calls these stations will receive at 8O kilocycles and transmit at 100 kilocycles. Likewise, on outgoing calls stationsD and E may be arranged to transinit at il() kilocycles and receive at 70 kilocycles, in which case on incoming calls these stations will receive at 110 kilocycles and transmit at 7 0 kilocycles. The repeater R-W, shown in Figs. 3 and 4, is arranged Y to transmit or receive `waves of the frequencies assigned to stations A and B, for eX- i ample 8O and'lOO kilocycles, While repeater .l-E, shown in Figs. 5 and 6, is arranged to transmit or receive waves of the frequencies assigned to stations D and E, for example 70 and 110 kilocycles.

The following description of station A will be followed by a description of the repeater station and a vconsideration of the method of communicating between the several stations.

Station A.

The apparatus at stationv A comprises an outgoing channel and an incoming channel which are connected to the power line by means of a coupling circuit 12. This coupling circuit may be of the type disclosed in a copending application of W. V. Wolfe, Serial No. 664,147, filed September 22, 1923, permitting the transfer of high frequency waves between the apparatus and the power line with safety to the apparatus and the operator.

High pass and low pass filters 13 and 14 are connected to the coupling circuit 12 and are arranged to be included alternatively in either the outgoing channel or the incoming channel by means of apparatus hereinafter described. rlhesc filters, and the other filters employed in the system, may be designed in accordance with the principles set forth in U. S. Patent No. 1,227,113 to G. A. Campbell, issued May 22, 1917.

Telephone communication with a dist-ant station is effected by means of an operators telephone set 15 comprising a receiver which is connected to the incoming channel, and a transmitter which is arranged to be connected to the outgoing channel when the receiver is removed from the switch-hook. A selector key 16 controls the transmission of calling signals to the distant stations in a manner hereinafter described. rlfhis key may be of any suitable type such as the master impulse transmitting device disclosed in U. S. Patent No. 1,35 ,814 to J. C. Field, issued Getober 5, 1920,'which may be adjusted to transmit the proper code or combination of impulses for callingfany station on the line. A selector 17 associated with the receiving channel re spends to incoming impulses for controlling the operation of a bell or other signaling device 18. The selector 17 may be of the type disclosed in U. S. Patent No. 1,343,256 to J. C. Field, issued June 15, 1920.

The outgoing channel includes (1) an amplifying tube 19 for amplifying the outgoing speech currents; (2) an oscillator 20 designed to produce waves at two different high fre-- quencies, for example, 8O and 100 kilocycles per second; a constant current modulator including the tube 21 and associated circuits whereby the outgoing high frequency waves are modulated in accordance with speech; (4) an amplifying tube 22 whereby the waves produced by the oscillator 20 are amplified to sufficiently high power for transmission under normal conditions; (5) an auxiliary amplifier 23 and its associated circuit connections which may be connected into circuit to increase the power amplification for transmission under abnormal conditions as, for example, when a section of the line is down; and (6) a switch 24 which when thrown to the left connects the output circuit of amplifier 22 to filter 13 or 14 through transformer 25, and when thrown to the right connects the output circuit of amplifier 22 to the input circuit of auxiliary amplifier 23 through transformer 26, and connects the output circuit of auxiliary amplifier 23 to filter 13 or 14 through transformer 27.

It will first be assumed that line conditions are normal and that'the auxiliary amplifier 23 is not to be used, in which case the switch 24 is closed in the left position. Filament heating current is supplied to the tubes 19, 20, 21 and 22 under the control of a heavy duty relay having a high inductance winding which is adapted to be energized in a manner hereinafter described. The filament heating circuit extends from grounded battery 31, through the filaments of the tubes, right hand armature and contact of relay 30, to ground. Space current is supplied to the tube 19 through a circuit extending from the positive side of battery 32, through the primary winding of transformer 33, the plate, discharge path and filament of tube 19, resistances 34 and 35, to the negative side of battery 32. rlhe current flowing in this circuit produces a drop in potential across the resistance 34 which causes a negative biasing potential to be impressed upon the grid of tube 19. The oscillator tube 20 is supplied with space current through a circuit extending from the positive side of battery 32, through the primary winding of transformer 36, the plate, discharge path and lament of tube 20, filaments of tubes 21 and 19, resistances 34 and 35, to the negative side of battery 32..v A blocking condenser 37 prevents the plate potential from being applied to the grid of the oscillator tube 20. A condenser 33 which is permanently connected in the oscillator circuit determines the higher carrier frequency of 100 kilocycles vper second, and the addition of condenser 39 in parallel determines the lower carrier frequency of 80 kilocycles per second. The transformer 36 couples the. output circuit of oscillator 20 to the amplifier- 22 whereby the carrier waves are amplifiedl before being impressed upon the power line.

The carrier waves in the anode-cathode circuit of the amplifying tube 22 are modulated according to the constant current method of modulation disclosed in a copending application of R. A. Heising. Serial No. 525,906, filed December' 30, 1921. rlhe modulating tube 21 has its plate connected to the plate of the amplifying tube 22 through the primary winding of transformer 25. Space current is supplied to these tubes from the battery 32 through a retard coil 40.y The drop in potential across the resistances 34 and 35 due to the current flowing therein causes ay negative biasing potential. to be impressed upon the grid of the tube 2l. potential across the resistance 34, which is connected between the lilaincnt and grid of the amplifying tube 22, causes a negative biasing potential to be impressed upon the grid of this tube. The retard coil 40 oiiiers a high impedance to the speech and carrier currents and hence there Will be no change in current through this coil. Speech variations Which are impressed upon the grid of the modulating tube 2l, however, produce variations in the impedance oi the anode-cathode circuit of this tube. The condenser 4l serves to by-pass the carrier current around the retard coil 40, but offers an impedance such that changes in the impedance of the anodecathode circuit ofthe modulating tube 2l produce corresponding changes in the carrier current flowing in the anode-cathode circuit of the amplifying tube 22. Asa. resultl of this action the carrier Waves flowing in the output circuit of the amplifying tube 22 are modulated in accordance with the speech variations Which are impressed upon the grid of the modulating tube 2l. Both the upper and lower side bands produced by modulation are impressed upon the power line along with a component of carrier frequency. The condenser 42 serves to by-pass speech frequency currents in the output circuit of tubel9 around resistance 34 and 35. Condenser 43 ley-passes alternating currents around the plate battery 32, and condenser 44 b y-passes alternating currents around the lilament battery 3l.

It' line conditions are bad owing, for example, to a section of the line being down, the switch 24 is moved from the normal left position to the right position. The plates of the modulating and amplifying tubes 2l and 22 are then connected together through the primary Winding of transformer 26 instead of through the primary Winding oit transformer L5. rlille production and modulation ot' the carrier waves, however, is accomplished in the manner described above. Filament heating` current is supplied to the auxiliary amplifying tube 23 over a circuit extending Yfrom grounded battery 45, through the lila- .nlent of tube 23, switch 24, left hand armature and contact of relay 30, to ground. Space current issupplied to the tube 23 over a circuit extending from the positive side of batterry 46, through the primary Winding of transformer 2 ,the plate, discharge path and filament oi tube 23, resistance 47, tothe negative side of battery 46. The current flowing in this circuit produces a drop in potential across the resistance 47 which causes a negative biasing potential to be impressed ,upon the grid of the tube 23.

The drop in The condensers 48A and 49 serve to byepass alternating currents around battery 46 and resistance 47, respectively. VVhen the auxiliary amplifier 23 is connected in circuit, the amplified carrier and side band Waves are transmitted through transformer 27, filter 13 or 14, as the case may be, and couplingcircuit l2 to the power line l0.

rllhe incoming channel extends 'from the power line l0, through the coupling circuit l2,A to the high pass or louv pass filter 13 or 14, and includes, in addition to these ele-v ments, (l) a common collecting circuit 5l for receiving` unmodulated carrier waves, carrier Waves interrupted in accordance with the calling impulses, and. the side bands regresenting the carrier Waves modulated in accordance ivith speech; (2) an amplifier 52 for amplifying the received calling impulses; a rectifier 53 for rectii'ying the amplilied calling impulses; a demodulator 54 for combining the-,received carrier and side bands to reproduce the speech currents, andv an amplifier 55 for amplifying the speech currents. y

The amplifying and rectifying tubes 52 and 53 are supplied 'with Filament heating current through a circuitextending from the negative side of battery 5G, through resistances 57, 53 and 59, the lilaments of the tubes, to the positive side oi battery 55. Space current is supplied to the amplifying tube 52 through a circuit extendingv from grounded battery 60, through the primary Winding of transformer 6l, the plate, discharge path and grounded tilament oi tube 52. Space current is supplied to the rectitying tube 53 through a circuit extending from grounded'battery 60, through the winding ot relay 62, the plate, discharge path and groundedlilament oit tube 53. The grid ol" theamplitying tube 52 is made negative with respect to the negative end otitis filament due to the drop in potential produced by the lilament current in resistance 59. rEhe grid et the -rectitying tube 53`is made negative with respect to the negative end ot' its filament due to the drop in potential produced by the lilament current inresistances 58 and 59 and the filamentoi the amplifying tube 52. The relay 62 is a marginal relay and is not operated by the small current normally flowing in the anode-cathode circuit of the rectitying tube 53 when no calling impulses are received. The characteristics of the tube 53V and its associated circuits are such that the received calling impulses of carrier frequency arerectilied and caused to operate the relayA 62. The operation ot this relay causes the stepping up ot the selector V 17., as liereinaltm more .Tully described. The condenserl 63 by-passes the alternating current in the ou-tput circuit ot the tube The condenser 64 .by-passes alternating currents around the Winding of relay 62.

The demodulating and amplifying tubes 54 and 55 are supplied with filament heating current through a circuit extending from the negative side of battery 65, through resistances 66, ($7 and 68, the filaments of the tubes, to the positive side of battery 65. Space current is supplied to the tube 54 over a circuit extending from grounded battery GO, through the primary winding of transformer 69, the plate, discharge path and grounded filament of tube 54. Space current is supplied to the tube 55 over a circuit extending from grounded battery 60, through the primary winding of transformer 70, the plate, discharge path and grounded filament of tube 55. The grid of the tube 54 is made negative with respect to the negative end of its filament due to the drop in potential produced by the filament current in resistances 67 and 68 and the filament of the tube 55. The grid of the tube 55 is made negative with respect to the negative end of its filament due to the drop in potential produced by the filament current in resistance 68. A gain control potentiometer 71 is included in the input circuit of the den'iodulator tube 54. A condenser 72 in the output circuit of tube 54 serves to by-pass the high frequency carrier waves. The speech frequency waves in the output circuit' of the demodulator 54 are amplified by the amplifier 55 and transmitted through transformer 70, low pass filter' 7 3, resistance network 74, and transformer 75 to the receiver of the operators telephone set 15. The low pass filter 73 is designed to pass waves comprised within the speech frequency range and to suppress waves of other frequencies.

Repeater.

As pointed out above, the repeater shown in Figs. 3 to 6, inclusive, comprises a repeater RMV and a repeater l't--ll The repeaters R-JN and R-E each include transmitting and receiving apparatus similar to that provided at the stations, the apparatus at these repeaters being connected by means of relay circuits. The repeater thus resembles two stations plus the necessary connecting circuits, and, therefore, may be termed a two terminal repeater. The port-ion of the apparatus at the repeater which has been referred to as the repeater R--JW is connected to the power line 10 by means of a coupling circuit 112, and the portion of the apparatus referred to as repeater .lt-E connected to the power line by means of a coupling circuit 212. These coupling I circuits are similar to the coupling circuit 12 employed at station A.

On incoming calls the repeater R-VV receives on the lower carrier frequency of 8O kilocycles per second and transmits on the higher carrier frequency of 100 lilocycles per second and hence, as pointed out above, is

arranged to receive calls only from stations A and B. On incoming calls the repeater RE receives on the higher carrier frequency of 110 kilocycles per second and transmits on the lower c rrier frequency of 7() lilocycles per second so that this repeater, as pointed out above, can receive calls only from stations C and D. lNhen the repeaters R-`W and R--E are being used to repeat calls from station A or B to station C or D both repeaters transmit on their respective higher frequencies and receive on their respective lower frequencies. However, when the repeaters l-lV and R-l are being used to repeat calls from station C or D te station A or B both repeaters transmit on their respective lower frequencies and receive on their respective higher frequencies. lith this Varrangement the waves transmitted from either repeater cannot feed back into the receiving channel of the other repeater. At the repeater R-VV the high pass filter 113 is normally connected to the outgoing channel and the low pass filter 114 is normally connected to the incoming channel, while at the repeater R-E the high pass filter 213 is normally connected to the incoming channel and the low pass filter 214 is normally'connected to the outgoing chanel. l

Telephone communication between the repeater and either of the stations A or lis effected by means of an operators telephone set 115 comprising a receiver which is connected to'th-e incoming channel, and a transmitter which is arranged to be connected to the outgoing channel in a manner hereinafter described. A selector key 116 is employed to t ansmit calling signals when the operator at the repeater desires to communicate with the operation at either stat-ion A or B. A selector 117 associated with the receiving channel is operated according to a predetermined combination of impulses when either station A or B calls the repater. This selector controls the operation of the bell or other signaling device 118.

rlclephone communication between the repeater and either station C or D is effected by means of an operators telephone set 215 comprising a receiver which is connected to the incoming channel of the repeater R-E, and a. transmitter which is arranged to be connected to the outgoing channel of the repeater R-E as hereinafter described. The selector key 216 is employed to transmit calling signals for calling the operator at station C or D. The select-or 217 associated with the receiving channel of the repeater R-E is operated according to a predetermined code y Yline.

apprise the' operator atthe repeater as to which telephone set to use in answering the call.

D. Under such conditions, if communication has been established between stations C and D, for example, it is desirable to transmit a` busy signal to station A or B if the operator at either of these stations attempts to call either station C or D. According to the invention, if the operator at station A actuates his transmitting ykey 16 to call either station C or D when these two stations are busy, the opei'ator at station A will receive a busy tone ap-prising him that the -called station is busy. The busy tone, however, will not be applied to the line if the operator at station A calls either station B or the repeater RVV under the conditions just mentioned.

The transmission of the busy signal in a proper case is controlled bymeans including the auxiliary selectors 110 andv21() associated with the repeaters Ri-V and R-E, respectively. These selectors normally function to energize the filament circuits of the outgoing channels at the repeater. The selector 110,

for example, is arranged to be operated on,

a normal call to station A or. B, closing one of its associated contacts to control the operation of apparatus atfthe repeater. The selector 210 is arranged to close one of its contacts on a. normal call to station C or D and serves also to establish the proper circuit at the repeater. If a station, such as C or D, on one side of the repeater attempts to call la station, such as A or-B, onthe other side of the repeater when stations A and B or station A yor B and the repeater R-W are in service, the select-or 110 and other apparatus will function to apply a busy tone to the rlhe selector 210 will function in a similar manner when the `conditions are reversed, that is, when a. station at the left of the repeater attempts to call any station which is already in service at the right of the repeater.

The outgoing and incoming channels at the repeaters R-VV and R-E are similar to the corresponding channels at station Aand.

therefore, will not be described Vindetail exiy cept so far as it is considered necessary to understand the operation of the system. y Furthermore, since the outgoing and incoming channels at therepeaters 'Ri-'W7 and R--E are alike, reference will be inade'at this point only to the channels of the-repeater R-VV- The outgoing channel includes a speech fre- According to a feature of the invention, the p quency amplifier 119, an oscillator 120, a modulator 121, an amplifier 122, and an auxiliary amplifier 123 which may be connected to orl disconnected yfrom the circuitby means ofv switch 1211. The condenser 138`which is permanently 'connectedin the oscillator circuit determinesthe higher carrier frequency of 100 lilocyclesper second, and the addition- 0f condenser 139 in parallel determines the lower (parrier frequency of 8O kilocycles per secon The incoming channel includes an amplifier 152 for amplifying the received calling impulses, a rectifier 158 for rectifyingl these' impulses, a demodulator 15-l for demodulating the received speech modulated carrier waves, and an amplifier 155 for amplifying the speech currents. It will be noted that space current from the source 160 is supplied .to the rectifier tube 153 through the winding of'relay 162. This relay is marginal, like the 'relay 62 at station A, and is operated only in response to the rectified current flowing in the anode-cathode circuit of the rectifier tube 153.

@ailing and tal/@img through repeater station.

the method of calling and talking through the repeater station from station A to station C will first be considered.

The operator atstation A, in initiating a call to station C removes his receiver from the switclihoolr, tliereb operating-relay 76 over a circuit extending fly-oni grounded battery 77, tli'roughthe lower contact ofthe switchhook, winding of relay 7 6, to ground. Relay 76, in operating opens its Lipper Contact, thereby opening' an energizing circuit of the relay 78. Relay 79 is now energized over a circuit eX- tending from grounded battery 80, 'through the lower armature andcontact of relayv 76, windingof relay 7 9', to ground. RelayA 79 in closing itsv lowerinost contact'short-circuitsT relay 62 so that tliisrelay cannot be energized in responser tothe carrier waves subsequently.`

to be received. Relay 7 9 in closing its second yupperfcontact completes an energizing cirtact of relay '84, second upper contact andv armature of relay 79, windings of relays 81 and 82 in parallel, to ground. The operation of relay 81 serves to disconnect the high pass filter 13 from the outgoing channel and connect it to the incoming channel, and the opfV eration of relay 82 serves to disconnect the low pass filter 14 from the incoming channel and connect it to the outgoing channel.

rlhe relay 79 in closing its uppermost con- Y tact causes the operation of relay 30 over a circuit extending from grounded battery 85,

` through the uppermostcontact and armature of relay 79, winding ofrelay 30, to ground.

The condenser 86 and resistance 87 serve to protect the upper contact of relay 79y against sparking due to the high inductance of the winding of' relay 86. The energization of relay 86 causes filament heating current to be supplied to all the space discharge tubes in the outgoing channel, including the auxiliary amplifying tube 28 if' this tube is in service. The filament circuits were traced above in the description of the terminal apparatus. As shown in the drawing, the relay 88, which controls the connection of condenser 89 in the circuit of the oscillator 26, is energized over a circuit extending from grounded battery 89, through the Winding of a marginal pole changing relay 96, winding of relay 88, uppermost back contact and armature of relay 84, to Y ground, the current flowing in this circuit, however, being insuliicient to operate the pole changing relay 96. Since the relay 88 is energizedthe condenser' 89 is connected in the oscillatory circuit of the oscillator 26 and hence the oscillator 26 producescarrier waves of 86 lrilocycles frequency. The carrier waves of 86 kilocycles frequency, generated by the oscillator 26, are transmitted through the lowpass filter 14 and coupling circuit 12 to the power line 16.

The repeater R--VV is normally arranged to `receive carrier waves of 86 kilocycles frequency. Accordingly, the carrier waves of 86 kilocycles frequency transmitted from station A pass through the coupling circuit 112 at the repeater, and are selectively passed by the low pass filter 114 to the common collecting circuit 151. Carrier waves of' this frequency will also pass into the receiving channel at station B which, like the receiving channel at station A normally has its low pass filter conf nected to the receiving channel. The carrier waves received in the circuit 151 at the repeater R--l/V are ramplified by the amplifier 152 and rectified by the rectifier 158, the rectified waves causing the operation of relay 162. The operation of relay 162 causes the energization of relay 168 over a circuit extending from grounded battery 164, through the winding of relay 168, front Contact and armature of relay 162, to ground.

The operation of relay 168 causes the ener gization of the marginal pole changing` relays 165 and 166. rl`he energizing circuit of relay v165l extends from grounded battery 167,

through the winding of relay 165, uppermost front contact and armature of relay 168, to ground. rllhe energizing circuit of relay 166 extends from grounded battery 168, through the winding of relay 166, second left hand armature and back contact of relay 169, uppermost front contact and armature oi relay 168, to ground. The operationof relays 165 and 166 causes an impulse of current from the source 182 to bev supplied tothe electromagnets of selectors 116 and 117. These selectors will thereupon advance one step but will immediately fall back to the starting position. lf either of these selectors happens accidentally to be held in an advanced position before the call is initiated, this single pulse will cause it to advance another step to a position from which it will promptly fall back to the normal starting position before the operator at station A actuates his calling key 16. A A

lt will be noted that relay 176 is normally energized over a circuit extending from grounded battery 167, through the winding of marginal relay 165, winding of relay 176, uppermost back Contact and armature of relay 168, to ground. Now, upon the operation of relay 168, the energizing circuit of relay 176 is opened at the uppermost back contact of relay 168, but the deenergization of relay 176 performs no useful function at this time.

The relay 162, in operating, completes an energizing` circuit for relay 171, from grounded battery 172, through the right hand winding of relay 171, inner right hand back con-v tact and armature of relay 174, front contact and armature of relay 162, to ground. The operation of relay 171 establishes an energizing circuit for the slow releasing relay 17 5, which extends 'from grounded battery 176, through the winding of relay 175, innerright hand armature and contactof relay 171, inner right hand back Contact and armature of relay 174, front contact and armature of relay 162, to ground. The slow releasing relay 177 is thereupon energized over a circuit extending from grounded battery 178, through the winding of relay 177, contact and armature of relay 175, to ground. rl`he operation of relay 177 causes the energization of relay 179 over a circuit extending from grounded battery 186, through the winding of relay 179, armature andl front contact of relay 177, to ground.

The operation of relay 171, asdescribed above, also establishes an energizing circuit for relay 288 of the repeater 13.16 at the repeater, which may be traced from grounded battery 281, through the winding of relay 288, right hand armature and' contact of resoA lay 274, outer left hand armature and contact of relay 171, to ground. The operation of relay 288 causes the energization of relay 286 over a circuit extending from grounded battery 285, through the uppermost contact and armature of relay 288, winding of relay 286, to ground. The energization of relay 286causes filament heating current to be supf plied to all the space discharge tubes in the outgoing channel olf the repeater RWE, including the auxiliary amplifying tube 228 if this tube .is inservice. The filament circuits are similar to the corresponding circuitsqirevously described at station A.

connection of the condenser 289 in the circuit of the oscillator 226, 1s energized over a cir1 ltk will be noted. that the relay 276, whichcontrols the V362, winding of relay 384-, togrounth y 384 in attracting itsy lowerinost armature recaen.-A

Since the relay 270 is energized, the con-V denser 239 is disconnected from the oscillatory circuit of the oscillator 220, and hence carrier waves of 110 lzilocycles frequency are generated by the oscillator 220.

Carrier waves of this frequency ordinarily would not pass to the power line since thelow pass filter 21e, ywhich is normally connected to theoutgoing channel of the repeater ft-ll, will not pass waves of this frequency. However, the operation of relay 283 inthe yinanner described above completes an energizing circuit for the relays 281 and 282 which control. the connection of the high pass and low pass filters to the outgoing' and incoming channels. This energizing circuit extendsfrom grounded battery 293, through the first-lower armature and contact of relay 263, the upper armature Aand contact of relay 283, windings of relays 281 and 282 in parallel, to ground. The loperation of relay 281 disconnects the low filter 214` from the outgoing channeland connectsit to the incoming channel, and the operation of relay 282disconnects `the high pass filter 213 kfrom the incomingY channel and connects it to the outgoing channel. The carrier waves of 110 kilocycles frequency, generated by the oscillator 220, therefore pass `through the high pass filter 213 and coupling circuit 212 to the power line 10.

r1he station C is normally arranged to receive carrier waves of 110 `kilocycles lfre,- quency, high pass filter 313 thereat normally being' connected to the incoming channel. Accordingly, the carrier `.waves of cycles frequency relayed from the repeater station, pass through the `coupling circuit 312 at station C, and areselectively passed by the lhigh pass filter 313 to the common collecting circuit 351. Carrier waves of this frequency,

of course, also pass into the receiving channel at station B. The carrier waves received in the circuit 351 at station C are amplified by the amplifier 352 and rectified by the rec-- 353, the rectified Waves causing the op titicr l l eration of relay The operation of relay 332 causesthe operation of relay 384 over a circuit extending from grounded battery 391, through the armature and rcontini-t of relay opens a short-circuit path around the relay 332 so that the relay 362 will .not be short-circuited when relay 37 9 is subsequently oper` ated. Relay 384' in attracting its rstlower 110 liilo I Relay armature opens the energizing circuit of the filter controllingvrelays 381 and 382 so that these relays will not beoperated upon thel subsequent operation of relay 379. Accord- .ingly, since therelays 381 and 382 remain deenergized, thelow pass filter L3111 remains connected to the outgoing channel and the high pass filter 313 remains connected tothe incoming channel. The relay 384 in opening its uppermost back contact, opens the energizing circuit of relay 370, thereby connecting the condenser 339 in the o cillatory circuit .of the oscillator 320. Furthermore, the relay 38l'closes its uppermost front contact, establishing .an energizing circuit for the pole changing relay 390 from grounded battery 389, through the winding of relay 390,

.uppermost front contact and armature of relay 384, to ground. The operation of relay 39() causes an impulse of current from the source 332 to be supplied to the electromagnet of selector 317. vThe selector will thereupon advance one stepr but will .immediately fall back to the starting position. The sequence of operations described above took place `when the operator at station A removed his receiver froin the switchhoolr. and

prior to the actuation of theselector hey 1,6. Tie operator at station A now actua-tes the selector key 16 which'isset to transmit the proper combination of impulses for calling station. C. The operation of selector key 1G Vshort-circuits the relay 88 each time a tooth nnpulsescauses the condenser 39 to be plated alternately lout of and into the oscillatory cir-` cuit of the oscillator 20. During the. operation of the selectorv keyl 16, the frequency generated by theoscillator 20is thus changed back and forth from the low carrier frequency of kilocycles per second to the'high carrier frequency of 100 kilocyclescper sccond,.and vice versa. e However,tlie high carrier frequency is not transmitted to the power line inasmuch as the low pass filter 13,.which is now connected between the outgoing chan- 1 nel and the power line, isdesigned topass Vonly the low carrier frequency and to suppress the high carrier frequency. The groups of impulses of the low 'carrier frequency of 80 kilocycles per second ai'e thus transmitted vto and impressed-upon the power line.

The operation'of the selector key 16 also f completes an energizing circuit for the pole changing' relay 90, from grounded battery 89, f

through the winding-.of 'relay r90, closed contact'of' selector-key 16', to ground. The energization of relay 90 causes an impulse of cur- 1 rent from the battery 32to be supplied to the i i electi'oinagnet of selector 17 which causes the selector to advanceone step. AWhen the cont-act of the selectorkey 16 is opened relayf 90 releases, which causes an impulse of current of opposite sign through the electromagnet of selector 17. The selector thus advances step by step in response to the transmitted impulses. The operation of relay 90 and the selector 17 at this time performs no useful function, but is usually provided for in systems of this general type so that relay 90 may also be used to relay calling impulses to a substation, not shown in the drawing, if this feature should be found desirable.

The lower frequency impulses of 8O kilocycles per second which are transmitted over the power line pass through the coupling circuit 112 at the repeater, and thence through low-pass lilter 1111 to the common collecting circuit 151, amplifier 152 and rectifier 153. These impulses, of course, also pass into the receiving channel at station B.' Each time the relay 162 is deenergized in response to the incoming impulses, the relay 270 of the repeater R-E is short-circuited through the outer right hand armature and contact of relay 171 and the back contact and armature of relay 162. That is, the alternate deenergization and energization of the relay 162, in response to the received calling impulses, causes the alternate deenergization and energization of the relay 270, causing the condenser 239 to be placed alternately into and out of the oscillatory circuit of the oscillator 220. The calling impulses received by the repeater R-W thus cause the frequency generated by the oscillator 220 of the repeater R-E to be changed back and forth from the high carrier frequency of 110 kilocycles per second to the low carrier frequency of 7 0 kilocycles per second. The low carrier frequency of lrilocycles per second, of course, is not transmitted to the power line inasmuch as the high-pass filter 213, which is now connected between the outgoing channel of the repeater R-E and the power line, is designed to` pass only the high carrier fre quency and to suppress the low carrier frequency. Groups of impulses of 110 kilocycles per second, corresponding in number and arrangement to the groups of impulses transmitted from station A, are thus transmitted to and impressed upon the power line by the repeater R-E The alternate 'deenergization and energization of relay 162, as described above, causes the alternate deenergization and energization of relay 163 which, inoperation, causes the alternate deenergization and energizatien of the pole changing relay 165 of the repeater RMTW. The relay 165, in operating, causes the selector 117 to advance step by step in the manner previously described. The relays 175 and 177, being slow releasing relays, remain Operatedr during the relaying of the impulses by the repeater, the relay 177 maintaining the relay 179 energized. The relay 171 is therefore locked up during the relaying. o-f

the impulses, through a circuit extending' from grounded battery 172, through the lefthand winding and innermost left-hand contact and armature of relay 171, right hand front contact and armature of relay 179, to ground. It is necessary to maintain the relay 171 energized. at this time since the energizing circuit of the relay 270 of the repeater R-E, which is alternately short-circuited during the relaying of the impulses, extends through the outer right hand armature and contact ofthe relay 171.

The calling impulses of 110 lilocycles frequency, which are relayed from the repeater, are received by the stations C and D. These impulses cause the selectors at stations C and D to step up, but only the selector at station C closes the proper contact. rlhe impulses which are received at station C cause the ale ternate deenergization and energization of relay 362 which, in turn, causes the alternate deenereization and energization of relay 384. The intermittent operation of relay 384 by relay 362 causes the intermittent operation of the pole changing relay 390, causing the selector 317 to advance step-by-step until it closes its Contact corresponding to the trans initted combination of impulses. lhen the selector 317 at station C stops at the properl contact, the relay 37 8 is energized over a. circuit extending from grounded battery 380,

through the uppermost armature and contact,

of relay 376,upper winding of relay 37 8,closed contact of selector 317, to ground. The rela-y 378 is thereupon locked up through a circuit extendin g fromgroundedbattery 380,through the upper armature and contact of relay 376, lower winding and upper contact and armature of relay 37 S, to ground. T he bell 318 at station C is new energized over a circuit eX- tending from grounded battery 392, through the lower contact and armature of relay 378, electrcinagnet of bell 318 to ground.

lt will be noted that the contact of selector' lcy 16 at the calling station A is opened at the conclusion of the dialing, that is. this contact is opened after the last tooth has passed the contact arm of the selector hey, and as a result carrier waves of SO kilocycles frequency continue to llow from the outgoing channel at station A to the power line. Carrier waves of 110 lrilocycles frequency are therefore impressed upon the power line from the outgoing channel of repeater R-E at the 'con-V cl usion of the dialing operation. The carrier waves of 110 kil cycles frequency which are thus received at station C maintain the relays 362 and 3811 energized during the subsequent communication between stations A and C. The final operation of relay 334 causes an additional impulse through the pole chang'- ing relay 390 which causes the selector 317 to advance another step from -which advanced position it falls back to the starting position.

www;

lll)

The calling impulses received at therepeater station in the manner described above, cause the intermittent operation of relays 165, 166, 265 and 266, thus causing selectors 117, 110, 217 and 210 to advance step-by-step along with the selectors at the other stations. The selectors 117 and 217,012 course, are arranged to respond only to theirown particular combinations ot impulses, and hence these selectors do not close their Contacts in response to the combination of impulses assigned to station C but promptly tall back to the starting position before the conclusion ot the impulses. The selector 110 also falls back to the starting position since this selector is operated only on calls to stations AVV and B. The combination of impulses transmitted from station A to station C, however, not only causes theseleetor 317 at station C to close its associated contact but alsocauses the selector 210 at the repeater R-VV to lclose one of its contacts. This is due to the factV that, as pointed out above, one of the contacts of selector 210 .corresponds in its code setting to the combination o' impulses assigned to the selector' 317 at station C. When the selector 210 at the repeater R-E closes its associated contact corresponding to the code setting ot' the selector 317 at station C, relay 28o s energized over a circuit extendingfrom grounded battery 287, through the winding of relay 286, closed contact of selector 210, to ground. Relay 274 is thereupon operated over a circuit extending `from groundedrbattery 288, through the lett hand winding of relay right hand armature and front contact oit relay 286, to ground. The operation ot' relay 274, however, performs no 'useful 'function `at this time. The operation of relay 285also causes the energization of relay 83 over a circuit extending` from 'grounded battery84,tlirough the winding'of relay 183. middle lett hand armature and'contactof relay 179,lett hand front contacts of relay 23o, t -ground. A holding circuit forthe re- 'lay 183 is thereupon established `from grounded battery 184, through, the winding, first lower armature and contact of relay 183,-left band armature and contactvoi:l relay 179to g und, so that relay 183 remains energized eren atteirevlay 286 is` deenergized when e selector 210 falls back to thestartish o' position. The operation of relayf183 es? .1 es an energizing circuit for relay 130,v '.cin @rounded battery 185, through the up-v discharge tubes in the` outgoing Since the, con-l @cycles frequency are .generated-by thisv oscillator and transmitted through the high-'pass filter 113 and coupling circuit 112 to the power line. l

When the operator at station C, upon hearing the bell,V responds to the call by lifting his receiver from the switchhook, the relay 376 is eiiergized over a circuit extending from grounded `battery 377, through the lower contact ot' the switchhook, winding of relay 376, to ground. Relay 376 in attracting its upper armature opens the holding circuit of relay 378, thereby decnergizing this relay and the bell 318. The relay 379 is energized over a circuit extending from grounded battery 380, through the lower armature and` contact of relay 376, winding ot' relay 37 9, to ground. The relay 330 is thereupon energized 'over a circuit which may be traced from grounded battery 385, through thev uppermost contact and armature ot relay 37 9, winding' ot' relay 330, to ground. The operation of relay 330 causes the lighting of the lilanients of the space discharge tubes in the outgoing channel atestation C. Since the energizing circuit of re-' lay 370 is now broken at the uppermost back.

contact ot relay 334, the condenser 339 isfconnected in the oscillatory circuit ofthe oscillator 320, and carrier waves of 70 kilocycles vfrequency generated by this oscillator are 4transmitted through the low-pass iilter 314 and coupling circuit 312 to the power line.v

The operators at stations A and C may now carry on a conversation over the power line.

Speech modulated carrier waves of 80 kilo# cycles frequency impressed upon the power .line at station A pass through the coupling circuit 112 lat the repeater R-lV and thence throughthe low-pass iilter 114 to the common vcollecting circuit 151. These waves are combined in the demodulator 154 withthe uninodulated .carrier component which isralso transmitted'over the line. The low frequency.

components of demodulation appearing in the output o t the demodulator 154 are amplified y by the amplifier v155v and transmitted through ylow-pass lilter 173, `back contacts and arma-- turesof relay 289,. to the input circuit of the ,speechy frequency. amplifier 219. The speech currents r`w iich are thus supplied tothe out. goingV channel of repeater R`-E arey caused Iv' to modulate the carrier Waves of 110 kilocycles frequencyproduced `by the oscillator .220, and the modulated carrier wavesr are transmitted over the powerv line to station C. A: portion olthewaves received at the repeatver-R--l/V is rectified to maintain relay 162 en- "ergized The speech modulated carrier Waves of 110 kilocyclesV frequencyA received at vstation Clare demodulated in theusual mannerand the speech frequency components of demodulation pass throughthe low-pass filter 373,

. resistance network 374 and transformer 375 tothe receiver of. the operators telephone set .y 315, Similarly, the speech modulated carrier Cil waves ot kilocycles frequency impressed upon the power line at station C are transmitted to the repeater R-E where they are demod'ulated in the usual manner, the speechv frequency components ot deniodulation passing through the low-pass lilter 273, the back contacts and armatures ot relay 189, to the input circuit of the speech frequency ampli- [ier 119. The speech currents which are thus supplied to the outgoing channel ot' repeater y a lousy indication to the attendant at the repeater which he may observe by lifting his rceiver without closing the switchhook contacts. It, however, the operator at the repeater should close the switchhook contacts in raising his receiver, there will be no intert'erence to the call because the operating paths ot relays 189 and 289, which maintain the closure betwen the incoming and outgoing repeater channels, are open at the upper back contact ot relay 177.- Upon the completion ot the conversation, the operators at stations A and C will replace their receivers upon the switchhooks, causing the system to return to the normal condition shown in the drawings.

OaZZz'w/g md tal/cmg to repeater.

When thel opera-tor at station A desires to call and converse with the operator at the repeater, he proceeds in the manner employed in calling station C except that the selector key 1G must irst be adjusted to transmit the particular code ot impulses which will cause the selector 117 at the repeater to close its associated contact. wWhen such a call hasv been initiated at station A, the operation of the apparatus at station A and the repeater is the saine` as that previously described up to the stepping up ot the selectors in response to the calling impulses. Only the selector 117 at the repeater stops on the proper contact, the other selectors throughout the system .telling haelt to the vstarting position. rlhe operation of selector 117 causes the energization ot relays 17d and 199. The energizing circuit ot relay 174i extends trom grounded battery 188, through the lett hand winding ol: relay 1711, lett hand armature and back contact et relay 186, closed contact of selector 117, to ground. The energizing circuit ot relay 1 90'inay be traced from grounded battery 191, through the middle contact and armature ot relay 192, upper winding of relay 190, closed contact ot seleccircuit extending trom grounded battery 198,

through the electromagnet oi bell 118, lowermost armature and contact ot relay 190, uppermost armature and contact ot relay 192, to ground. The relay 17 l remains operated during the communication between station A and the repeater, preparing a circuit tor the transmission of the busy signal in case station C or D should attempt to call station A or B while the transmission circuit at the lett oi the repeater is in service. The holding circuit ot relay 17 4 extends from grounded battery 188, through the right hand winding and inner front contact of relay 174, "iront contact oil relay 162, to ground. The transmission of the busy signal is described hereinafter in connection with a call trom station A to station B, and hence will not be discussed turther at this point.

`When the operator at the repeater removes the receiver of the telephone set 115 trom the switchhook, the local circuit ot the operators transmitter is closed, and the relays 192, 189 and 289 `are operated. rlhe energizingv circuit. ot relay 192 may be 'traced from grounded battery 194:, through the lower contact ot the switchhooli, winding ot' relay 192, lett hand back contactl ot relay 195, to ground. Inasmuch as relay 174 is energized, the energizing circuit 'of relay 171 is broken at'the inner right hand armature of relay 174, and hence relays 175, 177

and 179 are deenergized. Relay 189 is theretore energized over a circuit. extending from grounded battery 19d through the lower contact of the switchhook, lower winding ot relay 189, back contact ot relay 17 7, t-o ground. Relay 289 is energized over a vcircuit extending from grounded battery 194, through the lower contact of the switchhook, lower windingof relay 289, back contact ot relay 177, to ground. The operation ot' relay 189 opens the connection between the incoming channel ot the repeater R-E and the outgoing channel oi the repeater lit-W, and connects the local transmitting circuit ot the operators telephone set.115 to the input circuit ofV the speech 'frequency amplifier 119. The eperators at station A. and the repeater may now converse, station A transmitting at 89 kilocycles and receiving at 100 kilocycles, and the repeater R-N transmitting at 100 kilocycles and receiving at80 lzilocycles.- 1t will be noted that the operation ot' the relay 289 connects the transmitter et the operators tele-- phone set 215 to the input circuit of the speech frequency ampliiier 219 of the'repeater R-E, so that the repeater R-E is free to receive a call from station C or D while the call between station A and the repeater R-VV is in progress. If such a call is received by the repeater R-E when the operator at the repeater is engaged inconversation with the operator at station A he may, of course, an

Uallz'ng and talking t0 station 0a same side 0] repeater.

The operation of the system on a call from station A to station B will now be described. Following this description, the method of transmitting the busy signal will be considered.

The operator at station A proceeds to call station B in the manner employed in calling either station C or the repeater, except that the selector key 16 must first be adjusted to transmit the particular combination of impulses which will cause the selector at station B, corresponding to the selector 17 at station A, toclose its'associated contact. When such a call has been initiated at station A, the operation of the 'apparatus atV station A and the repeater is the same as that previously described up to the stepping up of the selectors in response to the callingV impulses. The carrier impulses of [8O kilocycles frequency impressed'upon the power line at station vA also enter the receiving channel at station B which is normally arranged to receive carrier waves of kilocyc-les frequency. These impulses cause the selector ai station B t-o close its associated contact, operating the bell at station B ina manner similar to that vpreviously described iii-connection with the operation at station C. l

The carrier waves of 80 kilo'cycles frequency alsojenter the receiving Vchannelof repeater R-l/V, causing `the operation of relay 162. The operation of relay 169l causes the energization of relays 163 and 171 as previously described. The operation of relay 171' establishes anenergizing. circuit for relay 175,' relaysV 177 and 179, in turn, also being operated. The operation of relay 171 also energizes the relay 283 of repeater B-E, whereby carrier waves of 110 kilocycles frequency are impressed upon the power line by the repeater for` transmission tostations C and D. rllhe intermittent operation of relay 162in response to the callingii'npulses receivedat repeater lit-WV causes the stepping up of vthe selectors at the repeater,

'and also causes the calling impulses to 4be relayed to stations C v and D in the manner previously described. y. Since the combination of impulses assigned to station B has been Y transmitted, the selector 11() at the repeater B-W closes one of its associated contacts upon the completion of the calling impulses. The relay 186 is thereupon energized over a circuit extending from grounded battery 197, through the winding of relay 186, the closed contact of the selector 110, to ground. The operation of relay 186 establishes an energizing circuit for relay 17 4,'from grounded battery 188, through the left hand winding of relay 174, left hand armature and front Contact of relay 186, to ground. A holding circuit is thereupon established for the re? lay 174, from grounded battery 188, through the right hand Winding and inner right hand front contact and armature of relay 174, front contact and armature of relay 162, to ground. The relay 174, in operating, opens its inner right hand back contact, thereby opening the energizing circuit of relay 171. Relay 171 is thereupon deenergized, in turn releasing relays 175, 177 and 179. Accord* ingly, the common relay equipment at the repeater and the transmitting and receiving apparatus ofthe repeater R-E, are restored f torn-ormal with the exception of relay 174 whichr remains operated to control the transmission. of the busy signal, should ,occasion require.- Then vthe operator at station B, hearing the bell, responds to the call by lifting his receiver from the switehhool, the kcoifmection between stations A and B is cont, ypleted and the operators at these'stations may `'tion C. The operation of the apparatus at station D issimilar to that previously described in coiniectionwitlr station A. When the operator -atstation D lifts his receiver- Vfrom the .'switchhook, carrier waves of 110 ,lilocycles frequency v.are impressed upon the power line. These waves pass into the 're-` ceiving channels of station C and repeater R'-E. The waves which are received at the repeaterR-E cause the operation of relay 262' which, in turn, kcauses the operation of l relays and 271. YWhen vtheoperator at 'station D actuates his selector key, the `Carrier waves which are impressed upon the power line are interrupted to transmitthe calling impulses. These impulses cause `the selectors at station C and therepeater to advancestep-by-step. The selector 317 at station C thereupon closes its associated contact, energizing relay 878 through the circuit y extending from grounded. battery 380 through the uppermost armature and contact of relay 376, upper winding of relay 378, the closed Contact of selector 317, to ground. TheY operation of` relayl271 at the repeater performs no useful function at this time. rl`he selector' 210 at the repeater R-E also closes one of its associated contacts, operating relay 286, which, in turn, establishes an ener- ,cjizing circuit for relay 27 l. rlhe operation of relay 274, however, performs` no useful function at this time. Although relay 170 at the repeater` l-ljlf is alternately deenergized and energized during the transmission of the calling` impulses, such transn'iission of implfilses will have no effect upon the call in progress between stations A and B, since the relay 52 at station A is short-circuited and the relay 02 at station B is already energized by the carrier current transmitted from station A. When the operator at station C, hearing the bell, responds to the call by lifting his receiver from the switchhoolr, the talking circuit between stations C and D is completed, and the operators at these stations may converse without interfering` With the call which is simultaneously in progress on the other side of the repeater.

lf the operator at station C, for example, originates a call to stat-ion A at a time when the stations and B are in service as described above, a busy signal Will be transmitted to station C. Under such condition, assume that the operator at station C initiates a call to station A. The carrier Waves of 110 kilocycles frequency which are impressed eeiying channel of repeater RE, causing the operation of relay 202. Relay 271 is Ahereupon energized over a circuit extending from grounded battery 201, through the left l back contact and armature of relay 274-, front contact of relay 262, to ground. Relay 175 is thereupon energized over a circuiteXtending from grounded battery 176, through the winding of relay 175, innermost left hand armature and contact of relay 271, inner right hand back contact and armature of relay 27 4,

front contact and armature of relay 262, to ground. The operation of relay 17 5 causes the operation of relay 177 which, in turn, operates relay 179. A holding circuit is therepon established for relay 271, from grounded battery 201, through the right hand Winding and inner right hand Contact and armature of rlay 271, right hand contact and armature ot relay 179,-to ground. Since the relay 17 l is operated, an energizing circuit is established for relay 169, which may be traced from grounded battery 202, through the Winding of relay 169, left hand contact and armature of y 2711, right hand contact and armature oi relay 271, inner right hand front contact and armature of relay 171i, front contact and armature of relay 162, which relay is ener- ;fjzaed due to the carrier waves being receiyed im station A, to ground. rlhe function of relay 169 is to transfer the selectors norinally associated with therepeater R-l/V to nnen the power line at station C enter the re-.

rand ivinding of relay 271, inner right hand the repeater R-E. Accordingly, when the operator atstation C transmits the proper combination of impulses for calling station il, the selector 110 closes one of its contacts, thereby energizing relay 180. lt Was pointed out above that the select-or 110 is provided with two contacts which correspond, respectively, to the codo settings of the selectors at stations il and l. rlhe operation of relay 186 establishes an energizing circuit for relay 195, which extends from grounded battery 203, through the winding of relay 195,

innermost right hand contact and armatureof relay 159, right hand armature and right hand front contacts of relay 180, to ground. Relay 283 at the repeater RE is thereupon energized over a circuit extending from grounded battery 284, through the Winding of relay 283, innermost left hand armature and contact of relay 17 9, inne lmost right hand front contacts and armature of relay 195, to ground. The operation of relay 283 causes the operation of relay 230, thereby energizing the filaments of the space discharge tubes in the outgoing channel of repeater R-E, 'l`he relay 268 is nowenergized over a circuit extending from grounded battery 254, through the Winding of relay 263, front contact and armature of relay 262, to ground. The uppermost back contact of relay 263 is therefore opened and hence relay 270, which controls the connection of condenser 239 `in the oscillatory circuit of the oscillator 220, is deenergize'd. Since the condenser 239 is connected in the oscillatory circuit of the oscillator 220, carrier Wares of kilocycles frequency are generated at the repeater R-E` and transmitted through the low-pass filter 21d and the coupling circuit 212 tothe power line.

The operation of relay 195 also establishes an energizing circuit for the tone producing relay 204, extending from grounded battery 205, through the right hand Winding, contact and armature of relay 20d, resistance 206, inner left hand contact and armature of relay 195right hand front contact and armature of relay 179, to ground.v W hen the re'- lay 204: attracts its armature, the energizing circuit through its right hand Winding .is broken, thereby deenergizing this relay and again closing its back contactand again energizing this relay. rllhe intermittent operation of relay 204 causes the intermittent interruption of the current through its right hand or primary Winding, therebycausing a. tone to be induced inits left hand or secondary Winding. The secondary circuit of relay 201i includes, in addition to the secondary f winding` of the relay, a battery 208 and a condenser 209, and may be tuned to produce a tone of, for example, 150 cycles per second. Y

The tone produced by relay 204i is transmitted through the outer right Vhand contact and armature of `relay 195, inner left hand contact 

